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The Main Sequence of a Star Explained

written by: Sean Fears•edited by: Jason C. Chavis•updated: 5/24/2010

Fusion is a star's gateway to bigger and better (or at least hotter!) things and the starting point for the formation of star systems, and hydrogen is a star's means for getting there. The underlying relationship between mass and luminosity is a sign of the process all stars have in common.

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    Creating a Star with Hydrogen and Gravity

    In the previous article, I mentioned that clouds of hydrogen don’t automatically fuse; they have to meet a certain minimum mass threshold and overcome the outward pressure generated by centripetal force, thermal expansion, and the positive charge of the nuclei, before they can become a star. Gravity is the force that jump-starts the process, creating sufficient densities and temperatures for the fusion process to begin. Insufficient concentrations of hydrogen may become the fuel for a successive generation of starbirth or result in the formation of a brown dwarf (a failed star between approximately twice the mass of Jupiter and a tenth the mass of the Sun). As has been observed in surveys of extrasolar planets, there are a wealth of “failed stars" out there.

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    Understanding the Main Sequence

    Once fusion has been initiated, the star balances the pressure of its outer layers with the energy generated within and achieves hydrostatic equilibrium. At this point, it is considered to have joined the main sequence. The best way to explain the main sequence is visually. If you look at thie chart below (called a Hertzprung-Russell or H-R diagram in honor of its creators) of stellar mass versus luminosity, you’ll notice a densely-populated band where most of the stars are located. Within this band, the luminosity of a star is dependent on its mass. While a star is “burning" hydrogen, this is where it resides. (Keep in mind that the mass of a star is like the fuel rods in a fission reactor, controlling the rate of the reaction. The larger the star, the faster the fusion process occurs and the more rapidly a star depletes its fuel.) Large stars don’t spend very long on the main sequence, while small ones may spend tens of billions of years on it.

    The mass of a star affects its core temperature (and its color, which is directly proportional to its temperature), which in turn determines its predominant fusion process and its structure. Very hot stars, like a pot of boiling water, have exceedingly strong convection currents that transport both energy and matter throughout the star’s interior, while smaller stars may have little or no convection occurring. Convection not only affects how effectively energy makes it to the surface - it also determines how well fresh fuel gets distributed within the star, which may subsequently affect how the next phase in a star’s life proceeds.

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    H-R Diagram
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    Image credits

    Hertzsprung-Russell diagram: by Richard Powell

Stars, Fusion, and the Main Sequence

Most people know that stars are thermonuclear furnaces, but there are a lot of details that escape notice. How do stars form? Why is it that all stars aren’t the same? What happens at the end of a star's life? Find out the answers to these and other questions in this series!
  1. What You Need To Know About Fusion
  2. The Main Sequence of a Star Explained